1. Field of the Invention
The present invention relates to an automatic focusing apparatus for a camera or the like.
2. Related Art
A typical conventional focus detection apparatus for a camera or the like employs a technique for causing an accumulation type sensor such as a CCD sensor to receive a beam passing through a photographing lens, photoelectrically converting an optical signal into an electrical signal, and performing predetermined processing of the electrical signal, thereby detecting a focus state of the photographing lens.
When the accumulation type sensor is used, an object brightness monitor sensor is arranged near the above sensor (focus detection sensor). An accumulation time of the focus detection sensor is controlled in accordance with an output from the monitor sensor, and the dynamic range of the photoelectrically converted signal must be matched with the range of a processing system.
When an object brightness level becomes low, the accumulation time must be prolonged. When the accumulation time is excessively prolonged in an accumulation type sensor, sensor noise called a dark current is increased, and the response characteristics of the focus detection are also degraded. In practical applications, the accumulation time is limited to a given predetermined length of time, and the photoelectrically converted signal is not directly input to the processing system but is amplified by an amplifier. An output from the amplifier is then input to the processing system.
With the above arrangement, when an amplification factor (gain) is increased, the accumulation time can be shortened to improve response characteristics of focus detection. However, a noise component included in the photoelectrically converted signal is also amplified to decrease an S/N ratio. Therefore, a gain cannot be excessively increased in a high-precision focus detection apparatus.
In a conventional focus detection apparatus for a single-lens reflex camera, two images of beams which pass through two different pupil regions of a photographing lens are received by a pair of sensor arrays and are photoelectrically converted. A relative positional displacement of the image signals as outputs from the pair of sensor arrays is obtained to detect an amount of out-of-focus (defocus amount) of the photographing lens. In this case, since only a brightness distribution of a specific area of the object space is detected by the pair of sensors, a defocus amount cannot be detected in an object having no brightness distribution in the specific area.
A plurality of sensor pairs and the corresponding focus detection optical systems are prepared to extract brightness distributions of the plurality of areas. Therefore, focus detection for many objects has been attempted, as disclosed in Japanese Patent Publication No. 59-28886, Japanese Patent Laid-Open No. 62-212611, and Japanese Patent Application No. 62-23489 filed by the assignee of the subject application.
When focus detection is performed using a plurality of sensors, a longer period of time is required than when focus detection is performed using one sensor. As a result, the response characteristics of focus detection are undesirably degraded.
Although the plurality of sensors are not so disadvantageous during automatic focusing control, they are disadvantageous during continuous photographing since it requires good response characteristics.